W. Gary Allread

The influence of psychosocial stress, gender, and personality on mechanical loading of the lumbar spine.

Study Design. The effects of psychosocial stress on muscle activity and spinal loading were evaluated in a laboratory setting. Objective. To evaluate the influence of psychosocial stress, gender, and personality traits on the functioning of the biomechanical system and subsequent spine loading. Summary of Background Data. Physical, psychosocial, and individual factors all have been identified as potential causal factors of low back disorders. How these factors interact to alter the loading of the spine has not been investigated. Methods. Twenty-five subjects performed sagittally symmetric lifts under stressful and nonstressful conditions. Trunk muscle activity, kinematics, and kinetics were used to evaluate three-dimensional spine loading using an electromyographic-assisted biomechanical model. A personality inventory characterized the subject’s personality traits. Anxiety inventories and blood pressure confirmed reactions to stress. Results. Psychosocial stress increased spine compression and lateral shear, but not in all subjects. Differences in muscle coactivation accounted for these stress reactions. Gender also influenced spine loading; Women’s anterior–posterior shear forces increased in response to stress, whereas men’s decreased. Certain personality traits were associated with increased spine loading compared with those with an opposing personality trait and explained loading differences between subjects. Conclusions. A potential pathway between psychosocial stress and spine loading has been identified that may explain how psychosocial stress increases risk of low back disorders. Psychosocially stressful environments solicited more of a coactivity response in people with certain personality traits, making them more susceptible to spine loading increases and suspected low back disorder risk.

Measuring trunk motions in industry: variability due to task factors, individual differences, and the amount of data collected.

The focus of this study was to determine the amount of data needed to ensure sufficient accuracy in estimating mean trunk motions of employees performing industrial manual materials handling tasks. Over 450 tasks were selected, in which the load weight and the vertical start and destination heights of the activity remained constant throughout the task. Data were collected as employees did their work at the job site, using the Lumbar Motion Monitor. Variance components were estimated in a hierarchical design and used to compute standard errors of mean trunk kinematic measures. These analyses found task-to-task variation to be much larger than the variability due to either multiple employees performing the same task or to repetitive movements within a task. Also, it was found that no significant reduction in the standard errors occurred when data were gathered for more than three employees and three repetitions of each task by an employee. This study indicates that the vast majority of variability in mean trunk motions is accounted for by the design of work tasks, and variations due to repeated cycles of a task or to employees are rather minor. It is also important as a basis for future work on modelling low-back disorder risk based on a jobs trunk kinematic measures.

Spine loading and probability of low back disorder risk as a function of box location on a pallet

It is widely believed that depalletizing operations in manufacturing and service environments substantially increase the risk of occupationally related low back disorders (LBDs). It has been established that the weight of the box lifted off a pallet can affect the risk of occupationally related LBD but few have considered the influence of the location of the box on the pallet (region) when assessing risk. Thus, the objective of this study was to assess spinal loading characteristics and the probability of high LBD risk as a function of box weight and its location on the pallet. Ten experienced order selectors were recruited from a local distribution center and were evaluated as they transferred boxes of different weights (40, 50, and 60 lb) from six different locations (regions) of a pallet to a pallet jack. Workers were monitored for their trunk motion characteristics as well as the electromyographic (EMG) activity of ten trunk muscles as they performed the task. Workplace factors as well as trunk kinematic and EMG information were used as inputs to: (1) a risk assessment model, and (2) an EMG-assisted model that was used to predict the three-dimensional spine loadings that occurred during the task. The results indicated that conditions where a worker must reach to a low level of the pallet increased spinal load and risk probability far more than changes in the weight of the box. Thus, spinal loads were significantly large in magnitude and would be expected to lead to an increase in low back disorders when workers lifted form the lowest layer of the pallet. The load moment was found to be strongly influenced by pallet region, which resulted in increased spinal loading and risk probability as the moment increased. This effort has also facilitated our understanding as to why spine loading increases under the various conditions studied in this experiment. Nearly all differences in spinal loading can be explained by a corresponding difference in coactivation of the trunk musculature. This in turn significantly increases the synergistic forces supplied by each muscle to the spine and results in an increase in spinal loading.

Shoulder Muscle Fatigue During Repetitive Tasks as Measured by Electromyography and Near-Infrared Spectroscopy

Objective: The objective of this study was to quantify shoulder muscle fatigue during repetitive exertions similar to motions found in automobile assembly tasks. Background: Shoulder musculoskeletal disorders (MSDs) are a common and costly problem in automotive manufacturing. Method: Ten subjects participated in the study. There were three independent variables: shoulder angle, frequency, and force. There were two types of dependent measures: percentage change in near-infrared spectroscopy (NIRS) measures and change in electromyography (EMG) median frequency. The anterior deltoid and trapezius muscles were measured for both NIRS and EMG. Also, EMG was collected on the middle deltoid and biceps muscles. Results: The results showed that oxygenated hemoglobin decreased significantly due to the main effects (shoulder angle, frequency, and force). The percentage change in oxygenated hemoglobin had a significant interaction attributable to force and repetition for the anterior deltoid muscle, indicating that as repetition increased, the magnitude of the differences between the forces increased. The interaction of repetition and shoulder angle was also significant for the percentage change in oxygenated hemoglobin. The median frequency decreased significantly for the main effects; however, no interactions were statistically significant. Conclusions: There was significant shoulder muscle fatigue as a function of shoulder angle, task frequency, and force level. Furthermore, percentage change in oxygenated hemoglobin had two statistically significant interactions, enhancing our understanding of these risk factors. Application: Ergonomists should examine interactions of force and repetition as well as shoulder angle and repetition when evaluating the risk of shoulder MSDs.

Biomechanical, psychosocial and individual risk factors predicting low back functional impairment among furniture distribution employees

BACKGROUND Biomechanical, psychosocial and individual risk factors for low back disorder have been studied extensively however few researchers have examined all three risk factors. The objective of this was to develop a low back disorder risk model in furniture distribution workers using biomechanical, psychosocial and individual risk factors. METHODS This was a prospective study with a six month follow-up time. There were 454 subjects at 9 furniture distribution facilities enrolled in the study. Biomechanical exposure was evaluated using the American Conference of Governmental Industrial Hygienists (2001) lifting threshold limit values for low back injury risk. Psychosocial and individual risk factors were evaluated via questionnaires. Low back health functional status was measured using the lumbar motion monitor. Low back disorder cases were defined as a loss of low back functional performance of -0.14 or more. FINDINGS There were 92 cases of meaningful loss in low back functional performance and 185 non cases. A multivariate logistic regression model included baseline functional performance probability, facility, perceived workload, intermediated reach distance number of exertions above threshold limit values, job tenure manual material handling, and age combined to provide a model sensitivity of 68.5% and specificity of 71.9%. INTERPRETATION The results of this study indicate which biomechanical, individual and psychosocial risk factors are important as well as how much of each risk factor is too much resulting in increased risk of low back disorder among furniture distribution workers.

Musculoskeletal disorder risk during automotive assembly: current vs. seated.

Musculoskeletal disorder risk was assessed during automotive assembly processes. The risk associated with current assembly processes was compared to using a cantilever chair intervention. Spine loads and normalized shoulder muscle activity were evaluated during assembly in eight regions of the vehicle. Eight interior cabin regions of the vehicle were classified by reach distance, height from vehicle floor and front to back. The cantilever chair intervention tool was most effective in the far reach regions regardless of the height. In the front far reach regions both spine loads and normalized shoulder muscle activity levels were reduced. In the middle and close reach regions spine loads were reduced, however, shoulder muscle activity was not, thus an additional intervention would be necessary to reduce shoulder risk. In the back far reach region, spine loads were not significantly different between the current and cantilever chair conditions. Thus, the effectiveness of the cantilever chair was dependent on the region of the vehicle.

Musculoskeletal disorder risk as a function of vehicle rotation angle during assembly tasks

Musculoskeletal disorders (MSD) are costly and common problem in automotive manufacturing. The research goal was to quantify MSD exposure as a function of vehicle rotation angle and region during assembly tasks. The study was conducted at the Center for Occupational Health in Automotive Manufacturing (COHAM) Laboratory. Twelve subjects participated in the study. The vehicle was divided into seven regions, (3 interior, 2 underbody and 2 engine regions) representative of work areas during assembly. Three vehicle rotation angles were examined for each region. The standard horizontal assembly condition (0° rotation) was the reference frame. Exposure was assessed on the spine loads and posture, shoulder posture and muscle activity, neck posture and muscle activity as well as wrist posture. In all regions, rotating the vehicle reduced musculoskeletal exposure. In five of the seven regions 45° of vehicle rotation represented the position that reduced MSD exposure most. Two of the seven regions indicated 90° of vehicle rotation had the greatest impact for reducing MSD exposure. This study demonstrated that vehicle rotation shows promise for reducing exposure to risk factors for MDS during automobile assembly tasks.

Does personality affect the risk of developing musculoskeletal discomfort

Personality theory suggests that individuals can react differently to the same situation. The primary objective of this research was to study employees’ personality preferences as they related to manual materials handing jobs. The hypothesis tested was that those whose work preferences did not match the nature of their job requirements would report more psychosocial, physiological or psychological stress and strain compared with those whose personality preferences did match their jobs. A total of 133 employees from two distribution centres completed the Myers-Briggs Type Indicator and other inventories pertaining to their work environments. The results showed that, when employees’ personalities were better matched with the nature of their work environment, they generally reported less anxiety and physical discomfort and more job satisfaction and social support than those having a mismatch. This relationship was more prominent in the less physically demanding jobs, suggesting an interaction between physical workload factors and psychosocial influences. This research suggests that integrating knowledge of ones personality preferences with the physical and psychosocial demands of a job may increase ones understanding of the causes of musculoskeletal discomfort in industrial workplaces and aid ergonomists in designing jobs to better match individuals’ capabilities, limitations and work preferences.

Quantification of Hand Grip Force under Dynamic Conditions

A laboratory experiment was conducted to determine whether grip force capabilities are lower when the wrist is moved than in a static position. The purpose was to determine the wrist velocity levels and wrist postures that had the most significant effect on grip force. Maximum grip forces of five male and five female subjects were determined under both static and dynamic conditions. The dominant wrist of each subject was secured to a CYBEX II dynamometer and grip force was collected during isokinetic wrist deviations for four directions of motion (flexion to extension, extension to flexion, radial to ulnar, and ulnar to radial). Six different velocity levels were analyzed and grip forces were recorded at specific wrist positions throughout each range of movement. For flexion-extension motions, wrist positions from 45 degrees flexion to 45 degrees extension were analyzed whereas positions from 20 degrees radial deviation to 20 degrees ulnar deviation were studied for radial-ulnar activity. Isometric exertions were also performed at each desired wrist position. Results showed that, for all directions of motion, grip forces for all isokinetic conditions were significantly lower than for the isometric exertions. Lower grip forces were exhibited at extreme wrist flexion and extreme radial and ulnar positions for both static and dynamic conditions. The direction of motion was also found to affect grip strength; extension to flexion exertions produced larger grip forces than flexion to extension exertions and radial to ulnar motion showed larger grip forces than ulnar to radial deviation. Although, males produced larger grip forces than females in all exertions, significant interactions between gender and velocity were noted.

Shoulder Muscle Oxygenation during Repetitive Tasks

The purpose of this study was to quantify shoulder muscle oxygenation during repetitive shoulder exertions that were similar to motions found in automobile assembly tasks. Ten subjects participated in the study. There were three independent variables: 1) shoulder flexion angle; 2) frequency; and 3) force. The dependent measure was percentage change in muscle oxygenation for the anterior deltoid and trapezius. The results showed significant muscle oxygenation decreases for each of the main effects (shoulder flexion angle, frequency and force). The interaction of force and repetition was significant for the anterior deltoid, indicating that, as repetition increased the magnitude of the differences between the force levels increased. The interaction of repetition and shoulder angle was also significant. The results of this research illustrate that ergonomists need to consider the interaction of injury risk factors that may trigger musculoskeletal disorders of the shoulder.